The present invention relates to an integration construction between a boiler and a steam turbine and a method in preheating the supply water for a steam turbine and in its control.
The last heat face of a steam boiler before the smoke stack is either a flue-gas/air heat exchanger or an economizer. In the present application, a flue-gas/air heat exchanger is understood as a heat exchanger between flue gas and combustion air, in which the heat is transferred from flue gas to combustion air to preheat the combustion air. In the present application, an economizer is understood as a heat exchanger in which thermal energy is transferred from the flue gases to the supply water.
When a flue-gas/air heat exchanger is used, the supply water for the boiler can be preheated by means of bled steam from the steam turbine, whereby the efficiency of the steam turbine process is improved. A flue-gas/air heat exchanger, i.e. a heat exchanger, in which thermal energy is transferred from the flue gases directly into the combustion air, is usually not used in small steam power plants because of its high cost.
When a flue-gas/air heat exchanger is not used, the flue gases of the steam boiler are cooled before passing into the smoke stack using an economizer. In such case, the supply water cannot be preheated with the aid of the bled steam of the steam boiler because the preheating would raise the ultimate temperature of the flue gases and thereby lower the efficiency of the boiler.
In the economizer of a steam boiler, heat is transferred from the flue gases into the supply water. For a steam boiler, a steam boiler provided with a combustion chamber is used. A change in the temperature of the supply water in the economizer is lower than a change in the temperature of the flue-gas side. The temperature rise in the supply water is usually 40 to 50 percent of the respective lowering of temperature on the flue-gas side. Therefore, a difference in the temperature at the hot end of the economizer is considerably higher than at the cold end. This observation results in that, in addition to the heat obtained from the flue gases, heat from other sources can be transferred into the supply water. In a steam turbine process, it is advantageous to utilize bled steam for preheating the supply water.
The economizer of the steam boiler in a steam power plant is divided into two or more parts, the supply water being preheated in the preheaters of the high-pressure side provided between said economizer parts by the bled steam from the steam turbine.
With the aid of a connection, the integration of the steam boiler and the steam turbine process is made more efficient. By means of such arrangement, the flue gases of the steam boiler can be cooled efficiently simultaneously with enhanced efficiency of the steam turbine process.
The investment cost is lower than in an alternative provided with a flue-gas/air heat exchanger:
improved controllability and boiler efficiency
smaller boiler building
lower cost of the boiler.
When a flue-gas/air heat-exchanger solution is unprofitable, an improved process can be implemented with the structure since the use of bled steam can be increased.
The arrangement is preferred especially in an instance in which the combustion air of the steam boiler is heated in one or more steam/air heat exchanger(s) connected in series and utilizing bled steam.
In a prior FI patent No. 101 163, which corresponds to EP 0724683, of the applicant, the advantageous integration construction between the steam boiler and the steam turbine is known. It has proved to be useful that the temperature of the supply water flown through economizers positioned in the flue-gas duct. An amendment to the integration construction disclosed in the FI patent No. 101 163 is described in the present application.
It is disclosed in the present application that by controlling the by-pass flow of the first economizer of the preheater in a divided economizer and possibly by controlling the amount of bled steam of the preheater of supply water also in a by-pass connector, the integration degree of the steam turbine process can be controlled. The preheating is limited by the boiling temperature of the hottest economizer, and the lower limit is the closing of the bled. The control method exerts an efficient impact on the electricity production while deteriorating slightly the efficiency of the boiler when the use of bled steam exceeds the scheduled value. A change in the degree of integration is of the order 10%. A change in the efficiency of the boiler is 2 to 3% at most.
By controlling the flow portion of the supply water flowing past the economizer it is possible
(a) to control the ultimate temperature of the flue gas of the boiler as the power of the boiler changes and as the quality of the fuel varies
(b) to control the ultimate temperature of the supply water so that the ultimate temperature of the supply water after the economizer is as desired (being e.g. 10 to 20xc2x0 C. below the boiling temperature).
Particularly when a soda recovery boiler is in question, the flue gases are highly soiling and corroding, and therefore, the soda recovery boilers cannot be provided with a flue-gas/air heat exchanger. The flue gases of the boiler are cooled by supplying supply water at about 120xc2x0 C. into the boiler. The preheating of the combustion air is important because of the combustion of black lye and therefore, the combustion air is heated with the aid of plant steam, typically to about 150xc2x0 C.
The above integration is not optimal considering the steam turbine process and therefore, the electricity power obtained from a back-pressure turbine will be low. As regards the boiler, an optimal situation prevails when the temperature of the flue gases exiting the boiler is as low as possible and no excessive soiling and corrosion of the heat faces is taking place yet. When the supply water into the boiler is in constant temperature, the temperature of the flue gases varies in accordance with the power level, the quality of fuel and the soiling situation of the heat faces. An optimal temperature is reached only occasionally on partial powers.
As described above, the optimal manner of driving the boiler is reached by integrating a soda recovery boiler and the steam turbine process as follows. The combustion air is preheated, instead of the plant steam, with bled steams of the steam turbine to about 200xc2x0 C., and a connector is connected between the economizers positioned in the flue gas duct of the boiler from the supply water preheater using bled steam. By controlling the temperature of the supply water entering into the boiler with the aid of the amount of the bled steam passing through the by-pass duct into the preheater and/or by controlling simultaneously the temperature of the supply water so that the amount of bled steam entering into the preheater is controlled, the ultimate temperature of the boiler flue gases can be controlled as desired in all running situations.
The integration construction between a steam boiler and a steam turbine of the invention in controlling the temperature of the supply water of the steam turbine is characterized in what is presented in the claims.